Reduction of chromium content in slag during melting of stainless steel in electric arc furnaces

ABSTRACT

The present invention relates to a method for reduction of chromium content in slag during melting of stainless steel in electric arc furnaces where steel scrap and additive alloys are melted in an electric steel furnace, whereafter molten steel and slag are tapped into a ladle, removal of the slag from the steel in the ladle, transferring of the molten steel from the ladle to a converter wherein the steel is refined and where the chemical composition of the steel is adjusted by addition of alloying elements. According to the invention fine particulate ferrosilicon is added to the slag during the time interval between the slag and the steel are melted, but before the molten steel and slag is tapped into the ladle.

TECHNICAL FIELD

The present invention relates to a method for production of stainlesssteel.

TECHNICAL BACKGROUND

By scrap-based production at stainless steel, steel scrap and alloyingmaterials are melted in electric arc furnaces. The steel scrap can beboth stainless steel scrap and carbon steel scrap while the alloyingmaterials mainly consists of FeCr and nickel or nickel alloys. Aftermelting in the electric arc furnace the steel, which has a high contentof carbon, is tapped into a ladle where the slag is removed before themolten steel is transferred to a converter where it is refined in orderto obtain a preset carbon content and where the final chemical analysisof the stainless steel is adjusted by the addition of additive materialsbefore the steel is being cast.

By stainless steel it shall herein be understood steel having a chromiumcontent of 4% by weight or more.

During melting of steel scrap and alloying materials in the electric arcfurnace some oxygen is being added as a part of the raw materials andsome oxygen from the surroundings is being picked up by the melt. Thus apart of the easiest oxidizable elements in the steel scrap and in thealloying materials will be oxidized and form part of the slag. One ofthe most valuable elements which is easily oxidized is chromium and asthe slag from the steel furnaces usually is dump or placed landfills,the chromium content in the slag is lost. A high chromium content in theslag is further an environmental problem.

In order to avoid losses of chromium in the slag it is conventionalpractice to add silicon to the charge together with steel scrap andalloying elements during melting in the electric arc furnace. Silicon isadded in the form of lumpy ferrosilicon or in the form of othersilicon-rich alloys such as for instance SiCr.

Even if silicon is added in order to prevent oxidation of chromium, theslag may after melting of the charge, contain a high and varying amountof chromium oxide. This is due to a number of factors, such that theexact amount of oxygen which enters the melt in the arc furnace is notknown, thus making it impossible to calculate the correct amount ofsilicon to be added. By a too low addition of silicon the chromiumcontent in the slag will be unacceptably high, while a too high additionof silicon will give a too high content of silicon in the steel which istransferred to the converter and which in turn will give rise to anincreased amount of slag during the refining, increased lining wear inthe converter and increased refining time, which gives a lowerproductivity and a higher production cost. Another problem by additionof lumpy ferrosilicon together with the scrap is that one will have aninhomogeneous distribution of the supplied silicon. It may then not besufficient time for the supplied silicon to react with chromium oxide inthe slag. Sometimes this can give a slag with a very high viscositywhich can make it difficult to tap the slag from the furnace such thatthe slag has to be remelted in the electric arc furnace during the nextbatch and, in addition, one will obtain a crude steel with an extremelyhigh silicon content in the converter.

In addition to what is discussed above, it is a wish during melting ofstainless steel in electric arc furnaces to use so-called foaming slag.In order to obtain foaming slag, carbon-containing materials are addedto the slag whereby CO bubbles are formed in the slag by the reactionbetween carbon and oxides in the slag, for instance chromium oxide. Theformed CO bubbles results in foaming slag. Foamed slag is more reactivethan normal slag and due to its volume, it protects the furnace liningagainst heat from the arc. This means that the furnace can be run at ahigher power that results in an increase in the melting capacity of thefurnace. It has, however, been found that it can be difficult to startthe slag foaming process, especially if the slag has a high content ofchromium oxide.

DISCLOSURE OF INVENTION

By the present invention one has now arrived at a method for reductionof the chromium oxide in the slag during production of stainless steelin electric arc furnaces at the same time as a low content of silicon isobtained in the steel produced in the electric arc furnace. Finally, bythe present invention one can in a simple and reliable way ensure theformation of foaming slag during the melting process.

Accordingly, the present invention relates to a method for reduction ofchromium content in slag during melting of stainless steel in electricarc furnaces where steel scrap and additive alloys are melted in anelectric steel furnace, whereafter molten steel and slag are tapped intoa ladle, removal of the slag from the steel in the ladle, transferringof the molten steel from the ladle to a converter wherein the steel isrefined and where the chemical composition of the steel is adjusted byaddition of alloying elements, said method being characterized in thatfine particulate ferrosilicon is added to the slag during the timeinterval between the slag and the steel are melted, but before themolten steel and slag is tapped into the ladle.

According to a preferred embodiment of the present invention, a part ofthe fine particulate ferrosilicon is added to the slag as soon as moltenslag is formed in order to initiate foaming of the slag, while theremaining part of the fine particulate ferrosilicon is added shortlybefore the molten steel and slag are transferred from the electric arcfurnace to the ladle in order to reduce the content of chromium oxide inthe slag.

According to another embodiment of the method according to the presentinvention the fine particulate ferrosilicon is added to the surface ofthe slag.

According to another embodiment of the method according to the presentinvention the fine particulate ferrosilicon is injected into the slag.

According to another embodiment of the method according to the presentinvention it is added fine particulate ferrosilicon in an amountsufficient to reduce all chromium oxide in the slag to elementalchromium.

The fine particulate ferrosilicon has preferably a particle size between0.1 and 5 mm. Particularly good results are obtained by adding fineparticulate ferrosilicon having a particle size between 0.3 and 3 mm andwhich contains between 65 and 80% by weight of silicon. The fineparticulate ferrosilicon is preferably water granulated ferrosiliconhaving a particle size between 0.3 and 3 mm. Water granulatedferrosilicon consists of substantially spherical shaped particles whichare easy to transport by pneumatic transport.

It has surprisingly been found that by the method according to thepresent invention a major part of chromium in the slag can be recoveredat the same time as the steel which is tapped from the electric arcfurnace has a stable and low content of silicon. By addition of fineparticulate ferrosilicon to the surface of the slag or into the slag itis obtained a high contact area between the slag and fine particulateferrosilicon as the fine particulate ferrosilicon is easily distributedover the whole surface of the furnace. This gives a very fast reactionbetween silicon and chromium oxide in the slag. Further, the time neededfor melting of the fine particulate ferrosilicon is substantially lowerthan for melting lumpy ferrosilicon. It was further surprising to findthat if a part of the fine particulate ferrosilicon is added just afterthe slag is melted, initiation of slag foaming is obtained. It isbelieved that the reason for this is that the early addition of fineparticulate ferrosilicon reduces the amount of chromium oxide particlesin the slag.

Tests with injection of water granulated ferrosilicon having a particlesize between 0.3 and 3 mm in accordance with the present invention haveshown that the content of chromium in the slag can be reduced by atleast 50% compared to what is obtained by conventional practice andwithout increasing the silicon content in the steel.

What is claimed is:
 1. An improved method for reducing chromium contentin slag during production of stainless steel in an electric arc furnacefrom steel scrap wherein the production of the stainless steel includesthe successive steps of melting steel scrap and additive alloys in theelectric arc furnace to form molten steel and slag, separating themolten steel and slag, refining the molten steel in a converter whereinthe chemical composition of the steel is adjusted by addition ofalloying elements, wherein the improvement comprises: adding a fineparticulate ferrosilicon to the slag during the melting step and beforethe separating step to reduce chromium content in the slag.
 2. Methodaccording to claim 1 characterized in that said adding step comprises:foaming said slag by adding a portion of fine particulate ferrosiliconto the slag as soon as molten slag is formed; and subsequently addingthe remaining part of the fine particulate ferrosilicon shortly beforesaid separating step.
 3. Method according to claim 1, characterized inthat the fine particulate ferrosilicon is added to the surface of theslag.
 4. Method according to claim 1, characterized in that the fineparticulate ferrosilicon is injected into the slag.
 5. Method accordingto claim 1, characterized in that the fine particulate ferrosilicon isadded in an amount sufficient to reduce all chromium oxide in the slagto elemental chromium.
 6. Method according to claim 1, characterized inthat the fine particulate ferrosilicon has a particle size between 0.1and 5 mm.
 7. Method according to claim 6, characterized in that the fineparticulate ferrosilicon has a particle size between 0.3 and 3 mm andcontains between 65 and 80% by weight of silicon.
 8. Method according toclaim 7, characterized in that the fine particulate ferrosilicon iswater granulated ferrosilicon.
 9. Method according to claim 2,characterized in that the fine particulate ferrosilicon is added to thesurface of the slag.
 10. Method according to claim 2, characterized inthat the fine particulate ferrosilicon is injected into the slag.